Theoretical Study Of Polarizability And Hyperpolarizability Of Molecule And Cluster

The dissertation is devoted to the study of polarizability of niobium cluster and hyperpolarizability of organic molecular-C50Cl10 from first principle. With the progress in density functional theory (DFT), first-principle calculation based on DFT has become a routine method for condensed matter theory, quantum chemistry and material science. In this dissertation, the concerned properties include geometry and electronic structure.In the first chaper, we generalize the relation between the nonlinear optical property and organic molecule, and introduce the concept of cluster. Also, mentioned is that the methods to calculate polarizability and hyperpolarizability.In the second chaper, the basic concept of DFT is introduced. The finite field (FF) is also presented.Starting from chapter 3, we focus on the nonlinear optical property of organic molecule C50Cl10. We study molecule C50Cl10 geometry and electronic structure, elucidate the distribution of π-electrons and demonstrate the effect of the energy gap (HOMO-LOMO energy gap) on the nonlinear property. Its static polarizability aand second-order hyperpolarizability γ are calculated by the use of a finite field approach. The diagonal components of polarizability a , especially the second hyperpolarizability y in the xy plane, which contains the Cl atoms, are much largerthan those in the z direction. The reason is mainly from the distribution of π electrons of the HOMO in the plane perpendicular to the z axis. We predict that C50Cl10 and its derivatives should be promising nonlinear optical materials.In chapter 4, we study the polarizability of niobium clusters including d electron. Based on DOS, the contribution to HOMO and LUMO is mainly from 4d electrons, not from 5s electrons. Considering from both the energy gap and permanent dipole moment, we find that if the energy gap of cluster composed of odd atoms is small, the permanent dipole moment of that is relatively big; otherwise, for cluster composed of even atoms, if the energy gap is big, its permanent dipole moment is relatively small. The average polarizabilities of clusters we calculated holistically seem to tend todescend with ascending in size of niobium clusters, and accord with the results of some experiments that the effective polarizabilities show oscillating; however quantitatively, the difference exists between our calculated values and experimental values mainly because that there may be different kinds of complicated electronic states in clusters of niobium but the stablest ground state only can be obtained by DFT, ferroelectric state and superconductivity etc. not considered.Then in chapter 5, we make conclusions about the contents above.